Conventionally, a refrigeration cycle apparatus employing a linear compressor which utilizes a mechanical elastic member or elasticity of a compressed gas as equipment for generating a compressed gas of a refrigerant, has been known. Concrete applications for such refrigeration cycle apparatus include an air conditioner for cooling or heating a room to keep a comfortable room temperature, a refrigerator for keeping its interior at an appropriate low temperature, and the like.
FIG. 11 is a diagram for explaining a linear compressor using a spring as an elastic member, which is used in the conventional refrigeration cycle apparatus.
A linear compressor 1 includes a cylinder section 71a and a motor section 71b which are adjacent to each other along a predetermined axis line. In the cylinder section 71a, a piston 72 which is slidably supported along the axis direction is placed. Further, in the cylinder section 71a, a piston rod 72a, an end of which is fixed to the rear side of the piston 72, is placed, and a support spring (resonance spring) 81 which applies a force to the piston rod 72a in the axis direction is provided on the other end of the piston rod 72a. 
Further, a magnet 73 is fixed to the piston rod 72a, and an electromagnet 74 comprising an outer yoke 74a and a stator coil 74b embedded in the outer yoke 74a is fixed to a portion of the motor section 71b which is opposed to the magnet 73. In this linear compressor 1, a linear motor 82 is constituted by the electromagnet 74, and the magnet 73 fixed to the piston rod 72a. The piston 72 reciprocates along its axis direction due to an electromagnetic force generated between the electromagnet 74 and the magnet 73, and elasticity of the spring 81.
Further, in the cylinder section 71a, a compression chamber 76 is formed, which is a closed space surrounded by a cylinder upper portion inner wall 75, a piston compression wall 72b, and a cylinder peripheral wall 77. An end of a gas inlet tube 10a for sucking a low-pressure refrigerant gas from a gas flow path into the compression chamber 76 is opened at the cylinder upper portion inner wall 75 and, further, an end of a gas discharge tube 10b for discharging a high-pressure refrigerant gas from the compression chamber, 76 to the gas flow path is opened at the cylinder upper portion inner wall 75. An inlet valve 79 and a discharge valve 80 for preventing back flow of the refrigerant gas are fixed to the end of the inlet tube 10a and the end of the discharge tube 10b, respectively.
In the linear compressor 1, when a driving current is supplied from a driving circuit (not shown) for the linear motor 82 to the linear motor 82, the piston 72 reciprocates in its axis direction, whereby suction of the low-pressure refrigerant gas into the compression chamber 76, compression of the gas in the compression chamber 76, and discharge of the compressed high-pressure gas from the compression chamber 76 are repeatedly carried out.
Meanwhile, as a method for controlling the refrigeration cycle apparatus, feedback control of the operation of the compressor which is a constituent of the refrigeration cycle apparatus on the basis of the thermal load condition of the apparatus is widely performed.
FIG. 12 is a diagram for explaining an example application for the refrigeration cycle apparatus, illustrating an air conditioner for cooling.
An air conditioner (refrigeration cycle apparatus) 50 is provided with an indoor unit 51 which is placed inside a room (indoor) to cool the room, and an outdoor unit 52 which is placed outside the room (outdoor) to discharge heat.
The indoor unit 51 includes an indoor heat exchanger (evaporator) 53 which performs heat exchange between the indoor air and the refrigerant, and absorbs heat from the indoor air; and a room temperature detector 54 which detects the temperature of the air to be sucked into the evaporator 53, i.e., the room temperature (the ambient temperature of the evaporator 53).
The outdoor unit 52 includes an outdoor heat exchanger (condenser) 55 which performs heat exchange between the outdoor air and the refrigerant, and discharges heat to the outside air; and a compressor 56 which is placed in a portion of a gas flow path Gp through which the refrigerant flows from the evaporator 53 to the condenser 55, sucks a low-temperature and low-pressure refrigerant gas from the evaporator 53 to compress the gas, and outputs a high-temperature and high-pressure gas to the condenser 55. Further, the outdoor unit 52 has an expansion valve 57 which is placed in a portion of a liquid flow path Lp through which the refrigerant flows from the condenser 55 to the evaporator 53, and reduces the pressure of a high-pressure liquid refrigerant so that the refrigerant evaporates at a lower temperature. In FIG. 12, Lmf indicates the direction along which the liquid refrigerant flows in the liquid flow path Lp, and Gmf indicates the direction along which the gas refrigerant flows in the gas flow path Gp.
Hereinafter, the functions of the condenser 55 and the evaporator 53 will be briefly described.
In the condenser 55, the high-temperature and high-pressure gas refrigerant flowing through the condenser 55 gradually loses heat and liquefies due to the air blown into the condenser 55, resulting in a high-pressure liquid refrigerant in the vicinity of the outlet of the condenser 55. This is equivalent to that the refrigerant radiates heat into the air to liquefy.
Further, the liquid refrigerant whose temperature and pressure are lowered by the expansion valve 57 flows into the evaporator 53. When the indoor air is blown into the evaporator 53 under this state, the liquid refrigerant takes a great amount of heat from the air and evaporates, resulting in a low-temperature and low-pressure gas refrigerant. The air which has lost a great amount of heat in the evaporator 53 is discharged as cool air from the blowoff port of the air conditioner.
As described above, in the air conditioner 50, a closed circuit for circulating the refrigerant is formed by the evaporator 53, the condenser 55, the gas flow path Gp and the liquid flow path Lp which are placed between the evaporator 53 and the condenser 55, the compressor 56 placed in the gas flow path Gp, and the expansion valve 57 placed in the liquid flow path Lp. The refrigerant sealed in the closed circuit is circulated by the compressor 56, whereby a known heat pump cycle is formed in the closed circuit.
As a method for controlling the amount of the circulating refrigerant, a method using a target temperature set on the air conditioner and the actual room temperature is generally employed (refer to Japanese Published Patent Application No. Hei. 9-68341).
FIG. 13 is a diagram for explaining a conventional refrigeration cycle control method for controlling an air conditioner for cooling.
In the conventional refrigeration cycle control method, the temperature inside the room cooled by the air conditioner (room temperature) is detected by an indoor unit suction temperature detector 60. As a concrete method for detecting the room temperature, a method of sensing the temperature of the indoor air by using a temperature sensor such as a thermocouple is used. Further, in a room temperature setting unit 61, a room temperature desired by the user is set as a target room temperature on the basis of an operation signal from the user. As a concrete method for setting a target temperature, a method of calculating a target temperature by processing a control signal from a remote controller of the air conditioner with a microcomputer is used. Then, a subtracter 63 calculates a temperature difference Tdiff between a room temperature Tdet detected by the indoor unit suction temperature detector 60 and a target temperature Tord set by the room temperature setting unit 61. A compressor rpm instruction unit 62 provides an instruction to the compressor 56 so that the rpm ωord of the compressor 56 becomes equal to an rpm according to the temperature difference Tdiff. To be specific, the rpm ωord of the compressor increases with an increase in the temperature difference Tdiff.
In the conventional refrigeration cycle control method, the rpm of the compressor 56 is changed in accordance with the difference between the temperature of the room to be cooled and the target temperature. Therefore, while highly efficient refrigeration cycle control can be carried out in a refrigeration cycle apparatus in which the amount of refrigerant circulating in the refrigeration cycle is set to a constant value according to the rpm of the compressor, it is difficult to carry out highly efficient refrigeration cycle control in a refrigeration cycle apparatus in which the refrigerant circulation amount is not determined by only the rpm of the compressor 56.
For example, in a compressor utilizing a conventional rotation type motor (rotation type compressor) such as a reciprocal compressor, a rotary compressor, or a scroll compressor, the volume of refrigerant to be compressed by one rotation of the motor is predetermined. Therefore, in a refrigeration cycle apparatus using such rotation type compressor, the amount of refrigerant that circulates in the refrigeration cycle is fixed to a constant value by the rpm of the motor of the compressor. Accordingly, in the rotation type compressor, highly efficient refrigeration cycle control can be carried out by controlling the rpm of the compressor.
On the other hand, in a refrigeration cycle apparatus using the above-described linear compressor, since the capacity of the compression chamber of the compressor varies, the volume of refrigerant to be compressed by one refrigerant compressing operation is not uniquely determined. Further, since the amount of refrigerant that remains in the compression chamber at the completion of the compressing operation is not constant, the amount of refrigerant circulating in the refrigeration cycle cannot be calculated from the stroke of the piston. As a result, the refrigeration cycle apparatus using the linear compressor cannot perform highly efficient refrigeration cycle control on the basis of the rpm of the compressor, i.e., the number of reciprocating motions of the piston per unit time.